Photographic emulsions consist of microcrystals of silver halide which are sensitive to light. After exposure to light, the emulsions are developed to form either a black and white or dye image, the density of which is generally dependent upon the amount of light that has exposed the microcrystals, and the inherent sensitivity of such crystals.
In an effort to improve the performance of photographic emulsions, the photographic industry has attempted to improve emulsion sensitivity. Much of its effort has focused on the treatment of photographic emulsion microcrystals with certain chemicals to enhance their sensitivity. Additional effort has been directed to modification of the morphology and/or composition of the microcrystals to make them more responsive to light, or more susceptible to development.
Dihydroheteroaromatic compounds have been reported as useful in the preparation of photographic emulsions. For example, in U.S. Pat. No. 5,192,654, dihydropyridines are utilized to ameliorate the effect of desensitization from certain spectral sensitizing dyes. In U.S. Pat. No. 3,893,862, 1,4 and 1,2-dihydropyridines are found to be useful as latent image amplifiers.
Through extensive research, the present inventors have found that additional types of dihydroheteroaromatic compounds may be incorporated into photographic emulsions to effectuate a desired photographic response. It has been found that certain dihydropyrimidines provide the photographic industry the opportunity to obtain improved sensitivity in photographic emulsions. With this discovery, there is the need to find a practical and effective way to manufacture such compounds.
The synthesis and stability of dihydropyrimidines have been extensively explored. Many of the known processes for synthesis involve the formation of a pyrimidine ring via the condensation of an acyclic carbonyl compound with a nitrogen containing compound. Typically, reactions of this sort involve high temperatures or strongly alkali conditions.
Other methods of forming dihydropyrimidines involve reduction of aromatic pyrimidines with complex metal hydrides or organometallic reagents that are costly for manufacturing. Other methods call for the addition of ammonia at -45.degree. C., a condition that is difficult and expensive to maintain during manufacturing. Still others require reactants that are not readily available or, because of the general lack of stability of dihydropyrimidines, involve processes that provide unacceptable yields.
Fused tricyclic pyrimidines have been reported by Richardson, "Fused Pyrimidines as Potential Antimicrobic Agents" Journal Of Medicinal Chemistry, Vol. 15, December 1972, pp. 1203-1206; Alaimo "The Synthesis of Some 4H-Pyrimido[2,1-b]benzothiazol-4-ones" J. Heterocyclic Chemistry, Vol. 10, October 1973, pp. 769-772; and Wade, "Reaction of 2-Aminobenzazoles with Dimethyl 2-Aminofumarate. Synthesis and Nuclear Magnetic Resonance Spectroscopy of 4-Oxopyrimido[2,1-b]benzazoles", J. Org. Chem., Vol. 44, No. 11, 1979 pp. 1811-1816, as well as in other chemical literature. Most of the syntheses disclosed involve the thermal condensation of 2-amino heteroazoles with unsaturated carbonyl compounds.
In U.S. Pat. No. 4,471,117, 3,4-dihydro-2H-pyrimido[2,1-b]benzothiazoles useful as antidepressants and anti-Parkinson's agents are disclosed. The compounds are prepared by known cyclization procedures involving heating of appropriately substituted alcohols or their halogenated analogs. In EP 0 465 728, mercapto substituted pyrimidines are disclosed as useful in the stabilization of silver halide emulsions comprising tabular grains. Such compounds are prepared by known methods.
It is known that certain alkynylamines can undergo a thermal Claisen rearrangement reaction to yield a mixture of tetrahydrobenzoquinolines and benzoquinolines. However, like many of the methods of forming dihydropyrimidines, such methods often require high temperatures, typically in the order of 100.degree. C. or more, or acidic or basic conditions in order for the reactions to occur. It is also known from Koch-Pomeranz "The Arrangement of Propargyl Phenyl Ethers Catalyzed by Silver Ions" Helvetica Chimica Acta., Vol. 56, No. 8, 1973, pp. 2981-3004 that propargyl phenyl ethers catalyzed by silver ions can undergo rearrangement reaction in benzene or chloroform to form the corresponding cyclic structure. Again, such reactions are typically carried out under conditions of high temperature. When carried out at lower temperatures, the product yield is severely curtailed. Additional metal catalyzed cyclization reactions involving alkynyl groups are disclosed by Balasubramanian and Nagarajan, "A Convenient Synthesis of 3-Methylthiazolo [3,2-1]benzimidazoles, 3-Benzylthiazolo[3,2-a]benzimidazoles, and 4-Phenylthiazeno[3,2-a}benzimidazoles", Synthesis, March 1976, pp. 189.
Despite the relative comprehensiveness of the art surrounding the synthesis of dihydropyrimidines, there has yet to be provided a satisfactory synthesis that can be used for the large scale production necessary in the photographic industry. Further, there has yet to be provided a synthesis that can be practiced at low temperature, with non-toxic solvents, and with few reactants.